A. Technical Field
The present invention relates to a water-absorbing agent and its production process and use, more particularly, relates to a water-absorbing agent of excellent urine resistance, especially, a water-absorbing agent which can always exhibit excellent absorption properties regardless of the kinds of liquids, such as urine, to be absorbed, and a production process for the water-absorbing agent, and further relates to uses of the water-absorbing agent, namely, to absorbent matters and articles, and still further relates to an absorption property measurement process by which absorption actions can easily and precisely be predicted when the water-absorbing agent and the absorbent matters and articles are practically used.
B. Background Art
In recent years, water-absorbent resins (water-absorbing agents) are widely used as constituent materials of sanitary materials, such as paper diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids such as urine and menstrual blood.
Known examples of the above water-absorbent resins are as follows: crosslinked polymers of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile copolymers or acrylamide copolymers, and their crosslinked polymers; and crosslinked polymers of cationic monomers.
It is said that the above water-absorbent resins should, for example, have the following properties: excellent water absorption amount and speed, the gel strength, the suction power to suck up water from a base material containing an aqueous liquid, upon contact with aqueous liquids such as body fluids. However, there are problems in that relations between these properties do not necessarily display positive correlations: for example, as the absorption capacity increases, some other properties such as liquid-permeability, gel strength, and absorption speed deteriorate.
As to a method for improving such water-absorption properties of the water-absorbent resin in good balance, an art is known, in which the neighborhood of the surface of the water-absorbent resin is crosslinked, and various methods have been proposed as such.
For example, methods are known, in each of which the following materials are used as the crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); monoepoxy compounds (JP-A-61-098121); epoxy compounds and hydroxy compounds as used jointly (JP-A-02-132103); alkylene carbonates (DE 4020780).
However, there are problems in that: the balance between the water-absorption properties is being improved by the above surface treatments, but when the water-absorbent resin is used for absorbent matters of diapers, the water-absorbent resin deteriorates with time, and the liquid-permeability or the gel strength falls, so urine leaks from the diapers. The deterioration of the water-absorbent resin occurs from the surface of the water-absorbent resin, and the soluble contents elute, and the liquid-permeability or the gel strength falls. Such a deterioration of the water-absorbent resin is considered to be caused by a very small amount of metal ion and by L-ascorbic acid as contained in urine.
By the way, the water-absorbent resin is powdery and therefore might contain fine powders of 100 xcexcm or less, and it is known to make a granulation by adding water for the purpose of improving the handling ability or the liquid-permeability in diapers. The granulation can prevent the powdering or improve the fluidity during moisture absorption.
However, there are problems in that the granulation by adding water to the surface-crosslinked water-absorbent resin facilitates the destruction of the surface-crosslinked layer. Especially, as to water-absorbent resins with high absorption capacity under load as desired in recent years, the elution of the soluble contents is prevented by crosslinking the neighborhood of the surface of water-absorbent resins with high absorption capacity, so the elution of the soluble contents cannot be suppressed in the case where the surface-crosslinked layer is deteriorated by substances such as L-ascorbic acid when absorbing urine. Therefore, there are problems in that when the water-absorbent resin is used for diapers, the liquid-permeability or the gel strength deteriorates, so urine leaks from the diapers.
On the other hand, as to uses of the water-absorbent resin, a variety of absorbent matters or articles using water-absorbent resins are proposed, wherein the water-absorbent resins jointly have a plurality of the aforementioned properties and exhibit excellent performance (water absorption properties) when used for sanitary materials such as paper diapers and sanitary napkins.
For example, the following are known: a water-absorbent resin comprising combinations of a gel capacity, a shear elastic modulus, and an extractive polymer content as are specified (U.S. Pat. No. 4,654,039); a water-absorbent resin with a water absorption amount or speed and a gel strength as are specified, and paper diapers and sanitary napkins using this water-absorbent resin (JP-A-60-185550, JP-A-60-185551, and JP-A-60-185804); paper diapers using a water-absorbent resin having a specific water absorption amount or speed and a gel stability (JP-A-60-185805); water-absorbent articles using a water-absorbent resin with a water absorption amount, a suction power, and a water-soluble content as are specified (JP-A-63-021902); water-absorbent sanitary supplies containing a water-absorbent resin with a water absorption amount, a water absorption amount under a load, and a gel fracture strength as are specified (JP-A-63-099861); paper diapers containing a water-absorbent resin with a water absorption amount and a water absorption speed under a load as are specified (JP-A-02-034167); a water-absorbing agent containing a water-absorbent resin with a water absorption amount under a load and a particle diameter as are specified (EP 339,461); a water-absorbing agent containing a specific or larger amount of water-absorbent resin with a water absorption speed and a water absorption amount under a load in a short time as are specified (EP 443,627); a water-absorbent combined material containing a specific or larger amount of water-absorbent resin with a deformation under a load and a suction index as are specified (EP 532,002); and an absorbent article using a resin with a pressure absorption index and a 16-hour extractability level as are regulated (EP 615,735).
In recent years, absorbent articles such as paper diapers are getting thinner and thinner, and the amount of the water-absorbent resin, as used for an absorbent layer of the absorbent articles, tends to increase. That is to say, as to the above absorbent layer, what has the weight ratio of 0.3 or more, particularly, 0.5 or more, of the water-absorbent resin to the total of the water-absorbent resin and the fibrous base material (this ratio might hereinafter be referred to as xe2x80x9cresin concentrationxe2x80x9d) is becoming the mainly current. However, it is becoming clear that there are, still, problems when the above already known resins with a variety of regulated properties are used for these absorbent articles having high resin concentration. That is to say, the water absorption properties of the absorbent articles are being improved by combinations of the above various properties, but it is being closed up that there are problems in that, depending on the composition of the liquid to be absorbed, the water absorption properties of the resins cannot be sufficiently displayed especially when the resin concentration in the absorbent articles is high. It is being said that there are problems in that, when the absorbent article is, for example, a paper diaper, the composition of urine varies with factors, such as users"" ages, taken food and drink, prescribed medicines, and in that the absorption action of the water-absorbent resin might therefore be greatly different from expectation.
A. Objects of the Invention
Therefore, an object of the present invention is to provide: a water-absorbing agent which undergoes little deterioration with time when absorbing urine and thus has excellent urine resistance; a water-absorbing agent which has not only excellent urine resistance, but also absorption properties that are stable to any composition of urine and show little change with time, and which is therefore used especially favorably for absorbent articles having high resin concentration; and production processes for these water-absorbing agents.
In addition, another object of the present invention is to clarify what absorption properties are needed for water-absorbent resins when the resin ratio is a specific value, and to provide an absorbent article using the optimum water-absorbent resin for each water-absorbent resin ratio, and to provide an absorbent matter and an absorbent article, both of which display an always stable high absorption amount, especially, a high absorption amount till the leakage occurs in a used state very near to practical use.
In addition, yet another object of the present invention is to provide an absorption property measurement process by which absorption actions can easily and precisely be predicted when the water-absorbing agent and the absorbent matters and articles are practically used, and which is very useful for producing a water-absorbing agent, absorbent matter, or absorbent article that displays excellent absorption properties.
B. Disclosure of the Invention
The present inventors studied and studied with encouragement to themselves and with great efforts to achieve the above object. As a result, the present inventors completed the present invention by developing new evaluation processes for (1) a deterioration absorption capacity under a load as seen using a specific liquid to be absorbed, (2) a deterioration absorption capacity under a load as seen after execution of specific procedure using a specific liquid to be absorbed, and (3) a deterioration absorption index under a load, and by finding that the above problems could be solved by a water-absorbing agent which exhibits a specific or larger value of absorption capacity or deterioration absorption index under a load in these evaluation processes. Parameter (1) above is not provided with the specific procedure, so it is hereinafter referred to as static deterioration absorption capacity under a load and includes four stages (1), (2), (3), and (4) in view of the largeness of the load, and particularly, stages (1) and (4) are important. Parameter (2) above is provided with the specific procedure, so it is hereinafter referred to as dynamic deterioration absorption capacity under a load.
Then, the present inventors found a process for obtaining a water-absorbing agent that displays the above specific absorption capacities or index (hereinafter, these might generically be referred to as parameters), in which an ion blocking or chelating agent including an amino polycarboxylic acid is preferably added to a water-absorbent resin by a specific method.
In addition, the present inventors studied and studied with encouragement to themselves and with great efforts about relations between the resin ratio in the absorbent matter and the physical properties of the absorbing agent. As a result, the present inventors completed the present invention by finding that the absorption amount, standing till the occurrence of the leakage in a used state very near to practical use, depends on specific relations as led from properties of the water-absorbing agent, such as absorption capacity under no load and the above new specific absorption capacities or index under a load, and from the resin ratio in the absorbent matter, and that the absorption amount of the absorbent matter or article in a used state very near to practical use increases if the water-absorbing agent and the resin ratio are selected so as to enlarge values of formulae of the above relations.
The water-absorbing agent, according to the present invention, can be any one of 1xcx9c3 below.
1. A water-absorbing agent, having an absorption capacity of 30 (g/g) or more under no load and static deterioration absorption capacity (1) of 20 (g/g) or more under a load.
2. A water-absorbing agent, having an absorption capacity of 30 (g/g) or more under no load and a dynamic deterioration absorption capacity of 20 (g/g) or more under a load.
3. A water-absorbing agent, having an absorption capacity of 30 (g/g) or more under no load and static deterioration absorption capacity (4) of 23 (g/g) or more under a load.
An absorbent matter, according to the present invention, comprises the above present invention water-absorbing agent and a fibrous base material, wherein the weight ratio of the water-absorbing agent to the total of the water-absorbing agent and the fibrous base material is 0.4 or more.
An absorbent article, according to the present invention, comprises:
an absorbent layer including the above present invention absorbent matter;
a liquid-permeable surface sheet; and
a liquid-impermeable back sheet.
An absorption property measurement process, according to the present invention, is characterized in that a liquid containing a reducible substance is used as a liquid to be absorbed in a process for measuring at least one absorption property selected from the group consisting of: absorption properties under a load of a water-absorbing agent; absorption properties of an absorbent matter of which the weight ratio of a water-absorbing agent to the total of the water-absorbing agent and a fibrous base material is 0.4 or more; and absorption properties of an absorbent article including the above absorbent matter.
A production process for a water-absorbing agent, according to the present invention, comprises the step of mixing an ion blocking agent and a surface-crosslinking agent, which is reactable upon a carboxyl group, with a water-absorbent resin having a carboxyl group.
Another production process for a water-absorbing agent, according to the present invention, comprises the steps of:
crosslinking the neighborhood of the surface of a water-absorbent resin which is obtained by polymerizing a monomer component including an unsaturated carboxylic acid in the presence of an internal-crosslinking agent; and
adding water and an ion blocking agent to the resultant surface-crosslinked water-absorbent resin, thus granulating the water-absorbent resin.
Yet another water-absorbing agent, according to the present invention, is obtained by a process including the step of adding to a water-absorbent resin at least one chelating agent selected from the group consisting of compounds of general formulae (1) and (2) and maleic hydrophilic polymers (including salts) (3),
wherein general formula (1) is: 
wherein: n, X1, and R1xcx9cR3 denote the following numbers and structures: 
and wherein general formula (2) is: 
wherein: m, X2, and R5xcx9cR8 denote the following numbers and structures: 
The above and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.